This section introduces aspects that may help facilitate a better understanding of the disclosure. Accordingly, these statements are to be read in this light and are not to be understood as admissions about what is or is not prior art.
In its various forms, graphene has garnered widespread interest for use in a number of applications, primarily due to its favorable combination of high electrical and thermal conductivity values, good mechanical strength, and unique optical and electronic properties. Of particular interest to industry are large-area graphene films for applications such as, for example, special barrier layers, coatings, large area conductive elements (e.g., RF radiators or antennas), and flexible electronics. A number of contemplated graphene applications have also been proposed for carbon nanotubes, since these two materials have certain properties that are comparable to one another. However, an advantage of graphene over carbon nanotubes is that graphene can generally be produced in bulk much more inexpensively than can the latter, thereby addressing perceived supply and cost issues that have been commonly associated with carbon nanotubes.
Graphene is composed of carbon atoms and is fabricated in atomically thin layers in which the carbon atoms reside at regular two-dimensional lattice positions. As a result of its high electrical and thermal conductivity properties, as well as its excellent mechanical properties, graphene has garnered a significant amount of interest. In particular, graphene represents an excellent conductive material allowing electrons to move substantially faster and in higher current densities than other competing materials, as well as having an excellent elasticity, flexibility, and transparency making graphene an excellent material for use in the semiconductor and electronics industries.
Various methods of synthesizing graphene are known. One such method is mechanical exfoliation which suffers from low throughput since it can only yield small size flakes with random distribution. Another method is based on SiC growth which poses difficulties in transferring graphene to other substrates. Other approaches such as chemical methods, e.g., graphene oxide reduction, have inadequate scalability and do not provide a suitable control over layer count.
Despite the fact that graphene is generally synthesized more easily than are carbon nanotubes, there remain issues with production of graphene in quantities sufficient to support various commercial operations. Scalability to produce large area graphene films represents a particular problem. The most scalable processes developed to date for making graphene films utilize chemical vapor deposition (CVD) technology. In typical CVD processes, a carbon-containing gas is decomposed at high temperatures into various reactive carbon species, which then deposit upon a suitable growth catalyst and reorganize to form a graphene film. It should be noted that in the present disclosure, the term deposition and growth or variation of these terms are used interchangeably and are intended to mean growth by deposition of graphene on a surface. In typical CVD graphene syntheses, a carbon-containing gas and a copper-containing substrate are heated to a high temperature (e.g., about 900° C.-1000° C.) that is just below the melting point of the copper (i.e., 1061° C.). Both metallic copper substrates and copper-coated substrates can be used (e.g., nickel or silicon carbide substrates coated with copper). It should be noted that while copper has been specifically identified here other metallic substrate or metallic coated substrates can also be used, e.g., Ni, Pt, and Ir. The CVD growth process can take place at either atmospheric pressure or a sub-atmospheric pressure. Due to the high temperatures employed in typical CVD processes, as well as the common use of reduced pressures during growth, scaling to afford graphene growth over large substrate areas can be expensive and complex. Further, since CVD growth processes often operate in the near-melting point regime of the substrate, substrate deformation can commonly occur, which can be undesirable for precision applications. CVD growth of graphene may not be possible at all on certain low melting substrates.
CVD is now one of the most reliable, scalable and rapid methods for large-area graphene growth. Thermal CVD generally involves growing graphene on a catalyst surface such as Cu, Ni, Ru, Pt, Ir, using a mixture of a hydrocarbon gas precursor and hydrogen at high temperatures (in the order of 1000° C.). After the growth, graphene can be transferred to a desired substrate by various processes, e.g., wet-etching of the metal catalyst. While thermal CVD can yield high quality graphene over a large area, the transfer step is not desirable since it can degrade the quality of the film by introducing defects and contamination. In addition to requiring a catalyst, in the thermal CVD process, the high temperature of growth also limits the type of growth substrates. To enable facile graphene integration into commercial devices, it is advantageous to reduce the growth temperature thereby increase the potential substrates and further avoid the necessity of the transfer operation from a catalyst to a substrate.
Therefore, there is an unmet need for a novel scalable approach for generating graphene that overcomes the above-discussed shortcomings.